The present invention relates to torches for electrical arc-welding in an atmosphere.
In the prior art, electrical arc-welding is performed either by striking an electrical arc in a normally inert shielding atmosphere between an electrode and the parts to be joined (the process being termed "TIG" (tungsten inert gas) or "MIG" (metal inert gas) depending on whether the electrode used is refractory or meltable) or by striking an electric arc between an electrode situated in a chamber and the parts to be joined, the chamber communicating with a passage through which the arc and a gas known as the plasma generating gas enter the chamber and from which they emerge to form the atmosphere for the arc. In this latter process, known as the "plasma" process, the wall of the chamber is formed by a part known as the "nozzle" which is vigorously cooled, generally by water circulating within it. The arc is severely constricted as a result of passing through the passage in the nozzle.
Both of these processes have drawbacks. The TIG or MIG process gives an arc the ends of which are unstable, both at the electrode and at the parts to be joined, with the result that the melted area on the parts to be joined becomes wider. In fact, with this process it is not possible to weld sheet metal thicker than 3mm in a single pass due to the fact that above a certain limit increasing the power of the torch results only in an enlargement of the melted area with practically no increase in penetration. What is more this process usually calls for a support at the back, on account of the fact that the melted area, being wide, needs to be supported.
The process in which a plasma arc is severly constricted by the passage in the nozzle and by a flow of gas which travels through this same passage at high speed allows a good, steady arc to be provided and considerable power to be transmitted, this power depending both on the welding current and on the throughput of plasma generating gas. It is easy to see that the plasma arc, due to the high energy density supplied and the high exit velocity of the gas, is particularly useful for cutting techniques; however, for this very reason it is more difficult to use for welding although it can be employed by using the so-called keyhole technique which consists in making a hole in the parts to be welded at right angles to the arc and in shifting this hole along with the arc, the hole closing up again at the rear by capillary action and thus forming the joint between the two parts. This method allows sheet metal up to 7mm thick to be welded and, in an improvement recently developed, the arc is constricted not by mechanical means by making it travel along a passage, but by pneumatic means by causing the pointed end of the electrode to extend beyond the neck of a passage which converges and then diverges and by bringing about the constriction solely as a result of the action of the jet of plasma generating gas. Generally speaking, the nozzle is liable to deteriorate as a result of secondary arcs; it needs to be cooled vigorously, generally by water circulating within it, so that, particularly when the arc is constricted by a passage through its nozzle, the nozzle will serve as the means to dissipate a considerable flow of heat. Furthermore, the structure of the torch itself is far more complicated, as also is the way in which it is supplied with electricity since on the one hand a high frequency supply is required to strike the arc and on the other hand the conditions which allow welding are set up in two stages, firstly by striking a so-called "pilot arc" between the electrode and the nozzle, with the nozzle serving as the anode, and then by transferring this arc to the parts to be welded so that the welding operation may begin, the nozzle being now electrically isolated.
It is an object of the invention to increase performance with the so-called "TIG" welding process while retaining the simple design of welding heads which employ this process.